DC off-set versus no negative feedback... - diyAudio
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Old 21st December 2010, 05:17 PM   #1
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Default DC off-set versus no negative feedback...

Hello:

I had a discussion with Balanced Audio Technology's designer regarding DC off-set.
He stated that the companies power amps typically provide about 100 mv DC off-set at the amplifier outputs.
I was surprised, because I had heard many, many, many discussions stating that 10 or below is outstanding, 20 -50 is within the safe zone and anything approaching 100 and above is unacceptable - possibly causing voice-coil heating and displacement and a loss in speaker performance and some other issues as well.
When I asked Victor about that he said that 10 or especially 5 and below is due to high use of global negative feedback and the 100 mv in his design is nothing to worry about.
I would be interested in your comments please.
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Old 21st December 2010, 06:17 PM   #2
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Yes, well, he would do, wouldn't he?

He also seems to have conveniently forgotten that the offset could be eliminated by AC (capacitor) coupling the output.

This is typical of the modern trend in amplifiers which is probably accurately described as 'design by dogma'.

The whole issue of amplifier design and specification has been muddied by the irresponsible behaviour of 'subjective' reviewers, or f*ckwits, as I prefer to call them.

The majority of cognoscenti would agree with your take on the issue, give or take a few mV, zero offset would be ideal.

What more can I say?

w
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Old 21st December 2010, 06:36 PM   #3
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Default DC off-set continued...

Hello wakibaki:

Well, referring to your response... "what more can I say", probably a little more at least.
Since I am not part of the cognoscenti, by formal document, that means I need to know a lot more:
In your opinion, or fact:
At what amounts would DC off-set become unsafe?
Is it true that the DC displaces the voice coil such that it is no longer centered at rest?
If so, is this similar to DC off-set in applications such as ..."remove DC off-set" in my Digital Audio Workstation's audio memory (shown in waveform editor), where the zero-crossing is not centered 50% between the pos/neg excursions?
I assume the effect is similar in that you are not getting correct potential between pos/neg limits, as well, in the speaker drivers too?

What about the claim that normal and or high levels of global negative feedback tend to provide low DC off-set and that that lack of global neg.feedback produces higher levels of DC?
I'm taking a guess here, but having seen specifications listing DC off-set from those two design types, I haven't, as yet, noticed a direct correlation.
However, since B.A.T. products have been selling product for awhile, and if their offset is not safe or could be better, should I assume they are not designed properly and thus not a smart purchase regarding offset (if and only your response should recommend what are safe levels)?
Anything else interesting to know about this?

Last edited by philmagnotta; 21st December 2010 at 06:45 PM.
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Old 21st December 2010, 07:01 PM   #4
Bigun is offline Bigun  Canada
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If your voicoil has a dc resistance of 6ohm then 0.1V is about 1.7mW.

So any thought in my mind about needing 100mV or lower is nonesense.
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Old 21st December 2010, 07:32 PM   #5
DF96 is offline DF96  England
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Any DC offset will cause voice coil displacement. Whether this matters or not depends on the amount, and whether the bass speaker is at its best at zero displacement. Lowest distortion need not necessarily coincide with zeo displacement, although it probably won't be far off. A few hundred mV should not do any harm, and won't cause much heating.

The real issue is that the standing current will displace the output stage (assuming normal PP). A few tens of mA offset will hardly be noticed in a Darlington output, but will push a complementary pair well away from the middle region. This may increase or reduce crossover distortion, depending on how well designed and accurately biassed the output is. For an unusual output stage it could be good or bad.
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Old 21st December 2010, 08:06 PM   #6
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Default DC off-set...

Quote:
Originally Posted by Bigun View Post
If your voicoil has a dc resistance of 6ohm then 0.1V is about 1.7mW.

So any thought in my mind about needing 100mV or lower is nonsense.
Bigun:
Thanks for the math.
In your example, it would appear to be a small issue: 100 mv (0.1V), that is.
I'm not qualified to say that in most cases, this would be true as the DIY member, DF96, replied below, but I'll ask some more questions directly to him.

DF96 responded:
... "Any DC offset will cause voice coil displacement. Whether this matters or not depends on the amount, and whether the bass speaker is at its best at zero displacement. Lowest distortion need not necessarily coincide with zeo displacement, although it probably won't be far off. A few hundred mV should not do any harm, and won't cause much heating.

The real issue is that the standing current will displace the output stage (assuming normal PP). A few tens of mA offset will hardly be noticed in a Darlington output, but will push a complementary pair well away from the middle region. This may increase or reduce crossover distortion, depending on how well designed and accurately biassed the output is. For an unusual output stage it could be good or bad."
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Old 21st December 2010, 08:13 PM   #7
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Quote:
Originally Posted by philmagnotta View Post
In your opinion, or fact:
At what amounts would DC off-set become unsafe?
The problem is really one of quantifying 'how much is too much.' A driver with small power handling will be more adversely affected than one with large power handling.

Quote:
Originally Posted by philmagnotta View Post
Is it true that the DC displaces the voice coil such that it is no longer centered at rest?
If so, is this similar to DC off-set in applications such as ..."remove DC off-set" in my Digital Audio Workstation's audio memory (shown in waveform editor), where the zero-crossing is not centered 50% between the pos/neg excursions?
I assume the effect is similar in that you are not getting correct potential between pos/neg limits, as well, in the speaker drivers too?
Yes. The voice coil is offset. Part of the driver excursion is lost. Part of the signal excursion is lost. The voice coil is heated while at rest. These effects are small, however.

Quote:
Originally Posted by philmagnotta View Post
What about the claim that normal and or high levels of global negative feedback tend to provide low DC off-set and that that lack of global neg.feedback produces higher levels of DC?
As I pointed out, an AC coupled amp with no global feedback could have no DC offset.

It really depends on topology, although it is true that negative feedback tends to mitigate against DC offset.

Quote:
Originally Posted by philmagnotta View Post
However, since B.A.T. products have been selling product for awhile, and if their offset is not safe or could be better, should I assume they are not designed properly and thus not a smart purchase regarding offset (if and only your response should recommend what are safe levels)?
Anything else interesting to know about this?
For myself, I wouldn't like to see more than a few mV of offset, not 10s of mV, and it is entirely possible that 100 or more will have an audible effect, although I have never knowingly run an amp with that much offset, that's based on what I've read and other people's observations.

100mV is probably not unsafe as such, given that it's ~2mW (DC), in typical modern domestic systems. A DCR of 6 ohms is frequently encountered in drivers. 2mW of AC is clearly audible, however, in many conditions.

Looking at Douglas Self's 'Audio power amplifier Design Handbook' we find: 'Most of my amplifier designs have assumed that a +/- 50mV offset is acceptable' ...'it could be argued that +/- 50mV is on the high side for a top-flight amplifier' ...'We might therefore hope to keep the DC output offset for the improved amplifier to within +/- 15mV without trimming or servos'. [My italics]

Obviously a design with trimmers or SOT components can see this improved upon, although a few mV of drift with time are unavoidable without a servomechanism. A servo can reduce offset to arbitrarily low levels.

As regards BAT, obviously the discussion of DC offset applies only to their SS offerings, the tube amps will be transformer coupled, thus being offset-free. For myself I would be happier to buy from a source that gives more prominence on their website to technical specifications, which are sketchy in the extreme, as opposed to reviews, of which there are many.

w
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Old 21st December 2010, 08:29 PM   #8
Elvee is offline Elvee  Belgium
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Speakers designed for certain types of acoustic enclosures have a very compliant suspension system as they rely on acoustic loading, and even a small DC voltage can cause a relatively large shift in the rest position of the voice coil.
100mV is probably safe, even for those speakers, but going much further doesn't seem very wise.
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Old 21st December 2010, 08:33 PM   #9
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Bottom line: 100mV offset=pis-poor design.
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Old 21st December 2010, 09:05 PM   #10
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Default DC off-set...

Quote:
Originally Posted by DF96 View Post
Any DC offset will cause voice coil displacement. Whether this matters or not depends on the amount, and whether the bass speaker is at its best at zero displacement. Lowest distortion need not necessarily coincide with zeo displacement, although it probably won't be far off. A few hundred mV should not do any harm, and won't cause much heating.

The real issue is that the standing current will displace the output stage (assuming normal PP). A few tens of mA offset will hardly be noticed in a Darlington output, but will push a complementary pair well away from the middle region. This may increase or reduce crossover distortion, depending on how well designed and accurately biassed the output is. For an unusual output stage it could be good or bad.
Hi DF96:
Thanks for pointing-out several ideas here.
Some additional questions please:
Speakers:
Are some drivers not at their best when at zero displacement?
Apart from the amplifier Xover distortion issue you latter mention, are you referring to speaker distortion relative to DC and driver off-sets?
How much, generally, should one find as a safe level?

Amps:
PP refers to push/pull?
Issues in displacement with a PP design are?...
... displacement of the A.C. waveforms at the speaker outputs?
As for the rest of the implications for amplifiers, one would have to know about the design, then check for DC.
Could you elaborate a little more please?
Thank you for the reply.
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